As is well known in the art, a thermal printhead utilizes a row of closely spaced resistive heat generating elements which are selectively energized to record data in hard copy form. The data may comprise stored digital information related to text, bar codes or graphic images. In operation, the thermal printhead heat elements receive energy from a power supply through driver circuits in response to the stored digital information. The heat from each energized element may be applied directly to thermal sensitive material or may be applied to a dye coated web to cause transfer of the dye by diffusion to paper or other receiver material.
The heat developed in each resistive heat element is a function of a number of factors including the voltage applied to the element, the thermal state of the element and the thermal states of the surrounding elements. For example, deviations in voltage across the resistive heat element cause variations in print density that are particularly noticeable in continuous tone graphical and pictorial images. Many different techniques have been devised to control the factors which determine the print quality. U.S. Pat. No. 4,736,089 (issued to Victor D. Hair on Apr. 5, 1988) discloses a switching regulator for a thermal printhead in which the printhead temperature is sensed by a voltage generating diode incorporated in the printhead. The diode voltage is fed back to control the reference voltage of a switching regulator power supply that provides power to the printhead.
U.S. Pat. No. 4,724,336 (issued to Takashu Ichikawa et al. on Feb. 9, 1988) discloses a power circuit for a thermal printhead in which the head resistance values are stored and the reference voltage of printhead power supply is selected from memory for each printhead element resistance. In this way, compensation is provided for the variations in the individual printhead element resistances. The arrangement, however, requires that the resistances of individual printhead resistances be measured and does not compensate for voltage or temperature variations.
U.S. Pat. No. 4,531,134 (issued to Frank J. Horlander on July 23, 1985) discloses a regulated voltage circuit for a thermal printhead in which the voltage at one electrode of each heat element is monitored and the lowest voltage is fed back to determine the current in a resistive ribbon printer via a differential amplifier control circuit. In this way, the energy to the heat elements is maintained above a predetermined minimum. U.S. Pat. No. 4,434,356 (issued to Timothy P. Craig et al. on Feb. 28, 1984) discloses a current drive circuit for a thermal ribbon printer in which the voltage at each ribbon resistance is monitored and used as a control input to a voltage regulator circuit that produces a head resistance drive voltage. In order to utilize either of these techniques in a multiple heat element printhead, it is necessary to access the electrodes of individual heat elements to obtain the required control voltage. None of the aforementioned patents solves the problem of voltage variations across printhead heat elements caused by internal printhead wiring resistances.
Many thermal printheads incorporate driver and other circuitry that control printhead operation so that it is difficult to obtain access to the electrodes of individual printhead resistive heating elements. It is relatively easy, however, to determine the voltage at the terminals of the printhead connectors. But the voltage across the printhead includes parasitic drops across power supply lines, interconnections and other wiring internal to the printhead. These parasitic voltage drops are proportional to the number of heat elements turned on for a print line. As a result, the parasitic voltage drops vary considerably as the number of selected heating elements changes. The varying heat element voltage produces noticeable variations in the density of the imprinted picture elements or pixels.
U.S. Pat. No. 4,774,528 (issued to Nobuhisa Kato on Sept. 27, 1988) discloses thermal recording apparatus in which the black density of pixels to be recorded by thermal recording elements are compared to reference density levels. A counter accumulates a value representing the number of pixels having density levels in certain ranges as a result of the comparison. The counter value is used to adjust the pulse width of energizing pulses to compensate for voltage fluctuations at the printhead heat elements due to the number of recording elements energized at one time. Adjustment of energizing pulse widths, however, is complex and does not yield sufficiently precise energy control to compensate for heat element voltage variations.
It is desirable to provide a relatively simple technique to accurately control the voltages across printhead heating elements without requiring access to the individual printhead elements.